Acquisition of thermoremanent magnetization in weak magnetic fields

By means of a phenomenological model of weak-field thermoremanent magnetiza tion (TRM) it is possible to reproduce the complex behaviour of TRM and par tial TRM (pTRM) to a good approximation. The model depicts a sample as a co llection of statistically independent magnetization elements, each of which possesses a definite blocking and a definite unblocking temperature. Moreo ver, for weak fields, linear dependence of remanence on applied field is as sumed for each magnetization element. These assumptions are sufficient to d erive to first order the experimentally found relations between the differe nt types of pTRMs. The kinematic equation, which is assumed to govern the c ooling process in multidomain (MD) material, is obtained from the model whe n mean-field interaction is taken into account. The phenomenological model is supported by two physical approaches. Using magnetic phase theory the co efficients that occur in the kinematic equation as derived from the phenome nological model can be physically interpreted for an isotropic ensemble of uniaxial MD particles. A rigorous micromagnetic analysis of the thermal equ ilibrium state proves the linearity of equilibrium remanence as a function of a weak applied field. A statistical argument extends this linearity to w eak-field TRM, which may be carried by non-equilibrium states.